First thing I would check is if your firewire card is true 100% OHCI compliant IEEE 1394 D/V. The hard part is finding out. Some claim to be and really aren't. If you look elsewhere on THIS site (way at top) there used to be a list of tested OHCI cards, DVD burners, etc..

Also, if you're using Vegas 5, under preferences, video tab drag the ? mark over and read to helpuful note. A sync offset may help.

Studdering can also happen if your PC is under powered, you have lots of applications running at once or if your set the preview mode too high. Dropping to a lower setting only effects the preview quality while you're looking at it during the editing process. It has no effect on the render quality.